Gas diffusion substrate

ABSTRACT

A fuel cell gas diffusion substrate has primary fibres, secondary fibres and one or more thermoplastic polymers for binding the primary and secondary fibres, characterized in that the secondary fibers are in the form of carbon nanofibers, and a gas diffusion electrode and membrane electrode assembly prepared therefrom are disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to a novel gas diffusion substrate for afuel cell, comprising primary and secondary fibres, and an electrodeprepared therefrom. The invention further relates to a process for themanufacture of the substrate and electrode.

Electrochemical cells invariably comprise at their fundamental level asolid or liquid electrolyte and two electrodes, the anode and cathode,at which the desired electrochemical reactions take place. A fuel cellis an energy conversion device that efficiently converts the storedchemical energy of its fuel into electrical energy by combining eitherhydrogen, stored as a gas, or methanol stored as a liquid or gas, withoxygen to generate electrical power. The hydrogen or methanol isoxidised at the anode and oxygen is reduced at the cathode. In thesecells gaseous reactants and/or products have to be diffused into and/orout of the cell electrode structures. The electrodes therefore arespecifically designed to be porous to gas diffusion in order to optimisethe contact between the reactants and the reaction sites in theelectrode to maximise the reaction rate. The electrolyte also has to bein contact with both electrodes and in fuel cell devices may be acidicor alkaline, liquid or solid, in nature. In the proton exchange membranefuel cell (PEMFC), whether hydrogen or methanol fuelled, the electrolyteis a solid proton-conducting polymer membrane, commonly based onperfluorosulphonic acid materials. The PEMFC is the most likely type offuel cell to find wide application as a more efficient and loweremission power generation technology in a range of markets includingstationary and portable power generation devices and as alternativeengines to the internal combustion engine in transportation.

In the PEMFC the combined laminate structure formed from the membraneand the two electrodes is known as a membrane electrode assembly (MEA).The MEA will typically comprise several layers, but can in general beconsidered, at its basic level, to have five layers defined principallyby their function. On either side of the membrane an anode and cathodeelectrocatalyst is incorporated to increase the rates of the desiredelectrode reactions. In contact with the electrocatalyst containinglayers, on the opposite face to that in contact with the membrane, arethe anode and cathode gas diffusion substrate layers. The anode gasdiffusion substrate is designed to be porous and to allow the reactanthydrogen or methanol to enter from the face of the substrate exposed tothe reactant fuel supply, and then to diffuse through the thickness ofthe substrate to the layer which contains the electrocatalyst, usuallyplatinum metal based, to maximise the electrochemical oxidation ofhydrogen or methanol. The anode electrocatalyst layer is also designedto comprise some level of the proton conducting electrolyte in contactwith the same electrocatalyst reaction sites. With acidic electrolytetypes the product of the anode reaction are protons and these can thenbe efficiently transported from the anode reaction sites through theelectrolyte to the cathode layers. The cathode gas diffusion substrateis also designed to be porous and to allow oxygen or air to enter thesubstrate and diffuse through to the electrocatalyst layer reactionsites. The cathode electrocatalyst combines the protons with oxygen toproduce water and is also designed to comprise some level of the protonconducting electrolyte in contact with the same electrocatalyst reactionsites. Product water then has to diffuse out of the cathode structure.The structure of the cathode has to be designed such that it enables theefficient removal of the product water. If water builds up in thecathode, it becomes more difficult for the reactant oxygen to diffuse tothe reaction sites, and thus the performance of the fuel cell decreases.In the case of methanol fuelled PEMFCs, additional water is present dueto the water contained in the methanol, which can be transported throughthe membrane from the anode to the cathode side. The increased quantityof water at the cathode requires removal. However, it is also the casewith proton conducting membrane electrolytes, that if too much water isremoved from the cathode structure, the membrane can dry out and theperformance of the fuel cell also decreases.

The complete MEA can be constructed by several methods. Theelectrocatalyst layers can be bonded to one surface of the gas diffusionsubstrates to form what is known as a gas diffusion electrode. The MEAis then formed by combining two gas diffusion electrodes with the solidproton-conducting membrane. Alternatively, the MEA may be formed fromtwo porous gas diffusion substrates and a solid proton-conductingpolymer membrane catalysed on both sides; or indeed the MEA may beformed from one gas diffusion electrode and one gas diffusion substrateand a solid proton-conducting polymer catalysed on the side facing thegas diffusion substrate.

Gas diffusion substrates or electrodes are employed in many differentelectrochemical devices in addition to fuel cells, including metal-airbatteries, electrochemical gas sensors, and electrochemical reactors forthe electrosynthesis of useful chemical compounds.

Traditionally, the gas porous substrates used in the PEMFC are based onhigh density materials such as rigid carbon fibre papers like TorayTGP-H-60 or TGP-H-90 (Toray Industries Inc.) or woven carbon cloths,such as Zoltek PWB-3 (Zoltek Corporation, 3101 McKelvey Road, St. Louis,Mo. 63044). These materials present a number of problems in terms ofcost, compatibility with high volume manufacturing processes andadaptability to the characteristics required for a wide range of celldesigns and operating regimes. Current estimates suggest that thesetypes of material are at least an order of magnitude too expensive formany applications, particularly for transportation, and their physicalstructure cannot be readily modified to ensure compatibility with therange of operating conditions envisaged. With existing carbon fibrepapers the rigidity of the material precludes the development of highvolume reel to reel processes, which offer the most attractive route forthe manufacture of the quantities of MEAs required. Carbon cloths couldbe used in reel to reel processes but their dimensional instability andtendency to fray at cut edges impose significant additional difficultiesin scaling up to high volume processes. For PEMFC's to becomecommercially viable power sources over a range of applications the gasporous substrate will require a manufacturing process capable ofproducing millions of square metres of material at very low cost andalso able to allow specific structural properties to be imported to thematerial for each application.

More recently, flexible electrode structures based on a porous substratecomprising a non-woven web of carbon fibres bound by a thermoplasticpolymer have been disclosed. The non-woven web is filled or coated withparticulate material such as carbon to achieve the required electricalconductivity. EP 0 791 974 demonstrates that electrodes based onnon-woven webs have comparable performances to structures based oncarbon fibre paper or cloth, without the drawbacks previously outlined.

Gas diffusion substrates based on filled/coated non-woven webs can beproduced with a wide range of specific structural properties, as shownin WO 00/47816 and WO 00/55933. The present inventors have now produceda gas diffusion substrate with alternative properties to those known inthe art. The properties are conferred by a type of non-woven web thathas different electrical conductivity and fibre packing properties tothe non-woven carbon fibres webs used in the prior art.

SUMMARY OF THE INVENTION

Accordingly the present invention provides a fuel cell gas diffusionsubstrate comprising primary fibres, secondary fibres in the form ofcarbon nanofibres, one or more thermoplastic polymers for binding saidprimary and secondary fibres, and a carbon based filler/coating materialwherein the loading of the carbon based filler/coating material is morethan 15 wt % based on the weight of the gas diffusion substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a vapour grown graphitic or semi-graphitic carbonstructure (tubular, solid, or intermediate in nature) that exist ascurved, intertwined entanglements or clusters;

FIG. 2 shows the results of cell potential over time for exemplaryembodiments of the present invention; and

FIG. 3 is a plot of the cell potential versus current density forexemplary embodiments of the present Invention.

DETAILED DESCRIPTION OF THE INVENTION

The primary fibres are carbon fibres as used in prior art non-woven webgas diffusion substrates. Suitably, the primary fibres have across-sectional dimension of at least 1 micron. The primary fibres aresuitably selected from the group consisting of longer fibres and shorterfibres, or a combination of longer and shorter fibres. The longer fibresare of average length greater than 3 mm and suitably have a maximumaverage length of 50 mm The preferred average length of the fibres is 5mm to 30 mm. The diameter of the longer fibres is typically in the rangeof 1 microns to 25 microns, suitably in the range of 1 microns to 20microns, and preferably from 1 micron to 15 micron, for example 4 to 12micron, such as 5 to 10 micron. The shorter fibres have an averagelength of less than 3 mm, suitably are of average length less than 2 mm,preferably less than 1 mm. The shorter fibres have a minimum length of50 microns, preferably 100 microns. The diameter of the shorter fibresis typically in the range 1 microns to 20 microns, preferably 1 micronsto 10 microns.

For the purposes of this patent carbon fibres can, at the fundamentallevel be defined as fibres having a high carbon content (typicallygreater than 95% carbon and more typically greater than 99% carbon) andan atomic structure that is generally graphitic in character. The degreeof order of the graphite structure and the ratio of graphitic toamorphous carbon will be determined by the starting material processingconditions and thermal treatment.

The nature of the atomic structure can vary significantly from highlygraphitic, but with high crystalline disorder (as in vitreous carbon) tolower levels of graphitisation but highly ordered structures typical ofhigh tensile fibres. Hence the physical properties such as the density,tensile strength, thermal conductivity and electrical conductivity canvary significantly.

The secondary fibres are those classed as carbon whiskers and fibrils(referred to herein as ‘nanofibres’) as supplied, for example, byApplied Sciences Inc., 141 West Xenia Avenue, Cedarville, Ohio45314-0579, U.S.A. or Nikkiso Co. Ltd., 43-2 Ebisu 3-chome, Shibuya-ku,Tokyo 150-91, Japan. These nanofibres are not conventional carbon fibresas used in prior art gas diffusion substrates. They have distinctproperties and are not merely very small conventional carbon fibres. Thefibres can be produced with much smaller fibre diameters and shorterfibre lengths than conventional carbon fibres. They also have adifferent morphology from the materials normally classified as carbonfibres. The carbon whiskers and fibrils (nanofibres) are typicallyvapour grown graphitic or semi-graphitic carbon structures (tubular,solid or intermediate in nature) that exist as curved, intertwinedentanglements or clusters, as shown in FIG. 1. The diameter of thenanofibres can typically be adjusted from 10 nanometres to 500nanometres and their length from 1 micron to 100 microns. Typical aspectratios range from 5 to as high as 1000. In the present invention, thenanofibres used suitably have a length less than 100 microns, preferablyless than 50 microns. The diameter of the nanofibres is suitably lessthan 500 nm, preferably less than 200 nm, more preferably less than 100nm

Preferably, the primary carbon fibres constitute between 10 and 90 wt %of the total weight of fibres and the secondary fibres constitutebetween 10 and 90 wt % on the same basis.

The primary and secondary fibres are held together by one or morethermoplastic polymeric substances (the “final polymer”). Depending onthe polymeric substance(s) used it may also contribute to the essentialelectrode structural properties in the gas diffusion substrate, such astensile strength, flexibility and control of the hydrophobic/hydrophilicbalance. Examples of such polymers include polytetrafluoroethylene(PTFE), fluorinated ethylene-propylene (FEP), polyvinylidene difluoride(PVDF), Viton A, polyethylene, polypropylene, ethylene-propylene. Thepreferred final polymer is PTFE or FEP.

In addition to the primary and secondary fibres, one or more continuousstrands comprising a plurality of tertiary carbon fibres may be embeddedwithin the gas diffusion substrate. The tertiary fibres in the one ormore continuous strands may be present in the form of a tow or yarn. Atow is an essentially parallel collection of synthetic fibrespreparatory to spinning, and a yarn is a continuous twisted strand oftwo or more fibres. When two or more continuous strands are embeddedwithin the substrate, the fibres in each continuous strand may be in theform of a tow or yarn, or a combination thereof.

The or each continuous strand(s) are made up of a plurality of tertiarycarbon fibres, and suitably comprise at least 100 fibres. The totalnumber of carbon fibres in each strand will depend on the requiredthickness of the substrate and the application for which it is to beused. The maximum length of the tertiary fibres is determined by thedimensions of the substrate and the orientation of the continuous strandwithin the substrate. For example, the continuous strand may extend fromone edge of the substrate to any one of the other edges, or thecontinuous strand may extend from one edge of the substrate to the sameedge. In all cases, the length of the tertiary carbon fibres will bedependent on the length of the continuous strand. The diameter of thetertiary carbon fibres is typically in the range of 0.2 microns to 25microns, preferably in the range of 2 microns to 20 microns. The finalprofile of the strand(s) within the substrate will depend on the numberand thickness of fibres in the strand and the final thickness of thesubstrate. The or each continuous strand is embedded within thesubstrate. When more than one continuous strands are present, eachcontinuous strand may be at an equal depth or at varying depths (i.e.variation in the z-direction) within the substrate, or a combinationthereof The continuous strand(s) may be applied at any orientation andat any spacing. The continuous strand(s) may also have applied to thesurface, or impregnated within the strand, a final polymer or polymerswhich may be the same as or different from the final polymer in thenon-woven carbon fibre gas diffusion substrate. Examples of suchpolymers include PTFE, FEP, PVDF, Viton A, polyethylene, polypropyleneand ethylene-propylene. The preferred final polymer is PTFE or PEP.

The carbon based filler/coating material comprises a mixture of aparticulate carbon and one or more polymers, the carbon suitably beingin the form of a powder. The carbon powder may be any of the materialsgenerally designated as carbon black such as acetylene blacks, furnaceblacks, pitch, coke-based powders and graphitised versions of suchmaterials. Suitably also both natural and synthetic graphites may beused in this application. Such materials may be used either alone or incombination. The particulate carbon, or carbons, in the basefiller/coating material are held together by one or more polymers. Thepolymeric materials used will contribute to the essential electrodestructural properties such as pore size distribution,hydrophobic/hydrophilic balance and physical strength of the gasdiffusion layer. It is preferable that the polymer is relativelyhydrophobic thereby rendering the base filler/coating material as awhole hydrophobic. Examples of such polymers include PTFE, FEP, PVDF,Viton A, polyethylene, polypropylene and ethylene-propylene. Thepreferred polymers are PTFE or FEP.

The carbon based filler/coating material may further comprise a catalystother than an electrocatalyst, for example a gas phase catalyticcomponent which is designed to remove contaminant gases in the fuel oroxidant feed streams, as for example carbon monoxide in the hydrogenfuel when this is supplied from a reformer. The gas phase catalyticcomponent comprises a supported or unsupported metal or mixed metalcatalyst suitably active for the oxidation of carbon monoxide to carbondioxide and which is isolated from the electrocatalytic component by theabsence of any proton conducting material within the structure.

The carbon based filler/coating material may further comprise a modifiermaterial or materials, which are added to the carbon basedfiller/coating material in order to change the water managementproperties of the structure. One or more modifier materials arehydrophilic in nature relative to the carbon based filler material.Preferably, the one or more modifier materials are based on carbon,glass, silica or ceramics, which may be hollow, porous or solid and aremore preferably essentially spherical or fibrous materials. By the termessentially spherical, we mean that the modifier material may bespherical, spheroidal, ellipsoidal or any shape which approximates to asphere, spheroid or ellipsoid. By the term fibrous, we mean that themodifier material is of a fibrous nature. That is the length is fargreater than the width or diameter; in general the fibres would not belonger than approximately 3 mm. Specific examples of the modifiermaterial include carbon wool, carbon nanofibres (as definedhereinbefore), quartz wool, silica micro-fibres, blown or sprayedceramic fibres, carbon micro-spheres, glass micro-spheres, colloidal orfumed silica and zeolites.

Further descriptions of the carbon based filler/coating material and themodifier material may be found in PCT Application WO 00/55933 which isincorporated herein by reference.

The loading of the carbon based filler/coating material is more than 15wt % based on the weight of the gas diffusion substrate, suitably morethan 25 wt % and preferably more than 35 wt %.

The gas diffusion substrate may be prepared in a two step processwherein the first step is the formation of a non-woven web comprisingthe primary fibres, the secondary fibres and the one or morethermoplastic polymers, and the second step is the filing or coating ofthe web with the carbon based filler/coating material.

The non-woven web may be made by either a wet-lay or a dry-lay processand may be made by a single individual process or by adapting acontinuous manufacturing process, such as paper making or felt making,to form a continuous web. In the case of a wet-lay process, the primaryand secondary fibres are dispersed as a suspension in, preferably water,to form a slurry. Also added to the slurry are one or more polymers (the“first polymer”), preferably hydrophilic polyrers, for examplepolyvinylalcohol (PVA). The first polymer may be in the form of fibres.Once the primary and secondary fibres and the first polymer areuniformly dispersed in the liquid, the resultant slurry is drainedthrough a suitable mesh in order to form a coherent layer of the web. Inthe case of a single individual process the fibres are deposited onto amesh in a conventional hand sheet maker. In the case of a continuousmanufacturing process, a continuous structure is formed by thecontrolled deposition of the slurry onto a moving mesh belt. The webso-formed by either method is dried in an oven to set the first polymer.If necessary the web is placed in a solution of the final polymer, eg athermoplastic polymer such as PTFE, which may or may not be the same asthe first polymer, allowed to dry and subsequently heat-treated to setthe final polymer. If it is not desirable for the first polymer toremain in the final web structure, it may be removed by this heattreatment or by an alternative appropriate process. In addition, anyundesirable residues may be removed by the heat treatment or by analternative appropriate process.

The flexible non-woven carbon fibre gas diffusion substrate may beprepared by taking a non-woven web, applying the carbon basedfiller/coating material to one or both sides of the web to form asubstrate and subsequently hot pressing. The majority of thefiller/coating material will be forced into the structure of thenon-woven web, although a small quantity may remain on the surface.Preferably a thin layer of the filler/coating material remains as asurface coating on one or both sides of the web.

The web may be in-filled by the base filler/coating material by anymethod known in the art. Such methods include screen printing, dipcoating, nip coating, spray coating and other coating processes known tothose skilled in the art.

Alternatively, the gas diffusion substrate may be prepared using a onestep process similar to that used to prepare the non-woven web. Forexample, the carbon based filler/coating material and where appropriatethe modifier material or materials may also be added to the slurry, anda continuous structure formed by the controlled deposition of the slurryonto a moving mesh belt and drying the formed gas diffusion substrate.The substrate formed may subsequently have a surface coating offiller/coating material applied to one or both sides.

A further aspect of the invention provides a gas diffusion electrodecomprising a gas diffusion substrate as hereinbefore described and anelectrocatalyst material. The electrocatalyst material is applied as athin layer to the surface of the gas diffusion substrate. Some of theelectrocatalyst material may penetrate slightly into the substrate, theremaining material forming a layer on the surface of the substrate. Theelectrocatalyst material comprises one or more electrocatalyticcomponents and a polymer. Suitable polymers include hydrophobicpolymers, such as PTFE and/or proton conducting polymers, such asperfluorinated sulphonic acids like Nafion®. The electrocatalyticcomponent is defined as a substance that promotes or enhances the rateof the electrochemical reaction of interest but remains unaltered by thereaction. The electrocatalytic component or components selected willdepend on the application for which the gas diffusion electrode is beingused. These may be, for example, a precious metal or a transition metalas the metal or metal oxide, either unsupported or supported in adispersed form on a carbon support; an organic complex, in the form of ahigh surface area finely divided powder or fibre, or a combination ofthese options. An example of a suitable electrocatalyst material isdescribed in EP 0731520.

A further aspect of the invention provides a membrane electrode assemblycomprising a gas diffusion substrate of the invention as hereinbeforedefined. Alternatively, the invention provides a membrane electrodeassembly comprising a gas diffusion electrode of the invention ashereinbefore defined.

Still further aspects of the invention include (i) a fuel cellcomprising a gas diffusion substrate according to the present invention,(ii) a fuel cell comprising a gas diffusion electrode according to thepresent invention, and (iii) a fuel cell comprising a membrane electrodeassembly according to the invention.

Other applications for which one or more of the embodiments of theinvention may be used, in addition to fuel cells, include, but are notlimited to, metal-air batteries, electrochemical gas sensors,electrochemical reactors for the electrosynthesis of useful chemicalcompounds and separator mats for batteries.

The following examples are illustrative but not limiting of theinvention:

EXAMPLE 1

1.0 g of chopped carbon fibres at a fibre length of 6 mm, and 1.0 g ofchopped carbon fibres at a fibre length of 12 mm (type RK 25 supplied byRK Carbon Fibres Ltd.) along with 0.7 g of carbon nanofibres (typePyrograf III, grade PR-24-AG supplied by Pyrograf Products, Inc.,Cedarville, Ohio, U.S.A.), and 0.5 g of polyvinyl alcohol fibres (typeMewlon SML supplied by Unitika Ltd., Oska 541, Japan) were dispersed in3 litres of water using a standard catering blender. The resultingdispersion was used to prepare a sample of non-woven sheet of size 330mm diameter (855.3 cm²) in a custom built sheet former similar ingeneral operation to a standard SCA sheet former, as supplied by ABLorentzen & Wettre, Box 4, S-163 93 Stockholm, Sweden. The sheet wasdried at 100° C. in air.

The non-woven carbon fibre sheet was dipped for 5 minutes in a solutionof 130 weight parts of a suspension of PTFE (ICI Fluon dispersion GP1,64 wt % solids suspension) in 2500 weight parts of water. The sheet wasdrained vertically until dry and the resulting sheet fired, in air, to385° C. for 15 minutes, to give a 33.8 wt % loading of PTFE.

The carbon based filer/coating material used to embed within the fibrenetwork was prepared by dispersing 47 weight parts of acetylene black(Shawinigan black from Chevron Chemicals, Houston, Tex., U.S.A.) in 1200weight parts of water. To this was added 3 weight parts of PTFE as adispersion in water (ICI Fluon dispersion GP1, 64 wt % solidssuspension) and the mixture stirred to entrain the PTFE particles withinthe carbon black. The resultant material was dispersed using a highshear mixer (Silverson L4R) to produce a smooth mixture.

The carbon based filler/coating material was pressed into the non-wovencarbon fibre sheet on one side, and leveled using a metal edge. Thesheet was then dried at 200° C. for 1 minute. A further thin layer ofthe material was applied to the same side of the sheet; the structurewas sandwiched between two sheets of filter paper and passed through aset of rollers to compact the layer. The sheet was then dried at 200° C.for 1 minute. Further additions of thin layers of the material wereapplied to the same side of the sheet with compaction and drying until aloading of 2.22 mg/cm² of carbon based filler/coating material within astructure containing 3.16 mg/cm² of carbon fibres was achieved. Theresulting gas diffusion substrate sheet was fired, in air, to 300° C.for 20 minutes.

A catalyst material used for forming the electrocatalyst layer on thegas diffusion substrate was provided by dispersing 100 weight parts of a40 wt % platinum on carbon black (Johnson Matthey HiSPEC™ FC40) in 30weight parts of Nafion® EW1100 (E I DuPont De Nemours & Co.) as a 9.5 wt% dispersion in water, prepared according to methods described in EP 0731 520. The particulate catalyst was dispersed in the Nafion® EW1100solution using a high shear mixer (Silverson L4R) to produce a smoothcatalyst ink.

A layer of the catalyst material was then applied to the top face of thefilled non-woven gas diffusion substrate, to give a loading of 0.4 mgPt/cm². The electrode formed the cathode of an MEA, with the platinumelectrocatalyst layer bonded to the membrane electrolyte face. The anodecomprised rigid carbon fibre paper (Toray TOP-H-90 from Toray IndustriesInc.) that had been treated with PTFE, with an electrocatalyst layercomprising a platinum/ruthenium on carbon catalyst (40 wt % Pt, 20 wt %Ru) at a loading of 0.45 mg Pt/cm². The membrane employed was Nafion®112 (E I DuPont De Nemours & Co). The single cell results, for initialconditioning of the MEA at 500 mA/cm² are shown in FIG. 2 and the cellpotential versus current density performance from the MEA for air andoxygen operation is presented in FIG. 3.

EXAMPLE 2

A further non-woven carbon fibre and nanofibre sheet, of size 330 mmdiameter (855.3 cm²), was prepared as for Example 1. The sheet wasteflonated and fired in the same manner to give a PTFE loading of 33 wt%.

The carbon based filler/coating material (prepared as described inExample 1) was pressed into the non-woven carbon fibre sheet from oneside of the sheet, and leveled using a metal edge. The sheet was thendried at 200° C. for 1 minute. A further thin layer of the particulatematerial was applied to the same side of the sheet; the structure wassandwiched between two sheets of filter paper and passed through a setof rollers to compact the layer. The sheet was then dried at 200° C. for1 minute. This process was repeated for the second side of the sheet.Further additions of thin layers of the particulate material wereapplied alternately to each side of the sheet, with compaction anddrying, until a loading of 3.43 mg/cm² of carbon based filler/coatingmaterial was achieved. The resulting gas diffusion substrate sheet wasfired, in air, to 300° C. for 20 minutes.

A catalyst material used for forming the electrocatalyst layer on thegas diffusion substrate was provided as in Example 1.

A layer of the electrocatalyst material was then applied to the one faceof the filled non-woven gas diffusion substrate, to give a loading of0.4 mg Pt/cm². The electrode formed the cathode of an MEA, with theplatinum catalyst layer bonded to the membrane electrolyte face. Theanode comprised rigid carbon fibre paper (Toray TGP-H-90 from TorayIndustries Inc.) that had been treated with PTFE, with anelectrocatalyst layer comprising a platinum/ruthenium on carbon catalyst(40 wt % Pt, 20 wt % Ru) at a loading of 0.43 mg Pt/cm². The membraneemployed was Nafion® 112 (E I DuPont De Nemours & Co). The single cellresults, for initial conditioning of the MEA at 500 mA/cm² are shown inFIG. 2 and the cell potential versus current density performance fromthe MEA for air and oxygen operation is presented in FIG. 3.

EVALUATION OF EXAMPLES

The MEAs (240 cm² active area) were tested at 80° C. and 300kPa absoluteH₂, air or oxygen gas pressure on anode and cathode. The conditioningcurves in FIG. 2 show acceptable performance for both electrodes. Thecell potential versus current density performance of both examples (FIG.3) are typical of the performance from an electrode produced by the bestcurrent gas diffusion electrode technology.

1. A fuel cell gas diffusion substrate comprising primary fibres,secondary fibres in the form of carbon nanofibres, one or morethermoplastic polymers for binding said primary and secondary fibres anda carbon based filler/coating material, wherein the loading of thecarbon based filler/coating material is more than 15 wt % based on theweight of the gas diffusion substrate.
 2. A gas diffusion substrateaccording to claim 1, wherein the primary fibres have a cross-sectionaldimension of at least 1 micron.
 3. A gas diffusion substrate accordingto claim 1 or claim 2, wherein the primary fibres consist of longerfibres.
 4. A gas diffusion substrate according to claim 1 or claim 2,wherein the primary fibres consist of shorter fibres.
 5. A gas diffusionsubstrate according to claim 1 or claim 2, wherein the primary fibresconsist of a combination of longer and shorter fibres.
 6. A gasdiffusion substrate according to claim 1 wherein the secondary fibreshave a diameter in the range of from 10 nanometres to 500 nanometres. 7.A gas diffusion substrate according to claim 1 wherein the secondaryfibres have a length in the range of from 1 micron to 100 micron.
 8. Agas diffusion substrate according to claim 1 wherein the primary carbonfibres constitute between 10 and 90 wt % of the total weight of fibresand the secondary fibres constitute between 10 and 90 wt % on the samebasis.
 9. A gas diffusion substrate according to claim 1 wherein one ormore continuous strands comprising a plurality of tertiary carbon fibresis embedded within the substrate.
 10. A gas diffusion substrateaccording to claim 1, wherein the carbon based filler/coating materialcomprises a mixture of a particulate carbon and one or more polymers.11. A gas diffusion substrate according to claim 1, wherein the carbonbased filler/coating material further comprises a catalyst other than anelectrocatalyst.
 12. A gas diffusion substrate according to claim 1,wherein the carbon based filler/coating material further comprises amodifier material or materials.
 13. A gas diffusion electrode comprisinga gas diffusion substrate according to claim 1 and an electrocatalystmaterial.
 14. A membrane electrode assembly comprising a gas diffusionsubstrate according to claim
 1. 15. A membrane electrode assemblycomprising a gas diffusion electrode according to claim
 13. 16. A fuelcell comprising a gas diffusion substrate according to claim
 1. 17. Afuel cell comprising a gas diffusion electrode according to claim 13.18. A fuel cell comprising a membrane electrode assembly according toclaim 14 or claim 15.